Aussies face worst heatwave in eight years

FORECASTERS have cautioned that there is "no reprieve" from the heatwave until at least the weekend as already record-breaking temperatures reach their peak.

Parts of Sydney will swelter into the 40s on Wednesday, just a precursor of a 45C high to come on Friday. Canberra could reach 41C today while 45C is likely in regional cities across New South Wales. On Tuesday, Hay, in the west of the state, almost touched 48C.

An extreme heatwave that poses a risk to even fit and healthy people is now in full swing in eastern parts of the state. The Bureau of Meteorology (BOM) has warned of "oppressive conditions".

Yes, it's summer and heat is no stranger this time of year - but this is something else.

The NSW Health Department has said it's now the worst heat the state has experienced for a prolonged period since 2011, when a heatwave increased the mortality rate by 13 per cent and hospital admissions by 14 per cent.

The unprecedented heatwave is reaching its peak across eastern Australia. Picture: BOM

Adelaide could peak at 40C today, but Melbourne and Tasmania will see some relief form the roasting conditions aided by sea breezes.

But you don't have to go far inland to see the mercury rise. Bendigo, Victoria could top out at 43C today.

Perth is fine and summery today but come the weekend it too will touch 40C.

BOM meteorologist Diana Eadie said much of the country remained stuck in heatwave mode.

"Temperatures are expected to climb into the low to high 40s - that's eight to 12 degrees above average.

"We've already seen some January maximum temperature records fall and we're likely to see many more before this event is over," she said.

"The humidity will lead to really oppressive conditions."

Authorities across the southeast have urged people to hydrate, check on vulnerable friends, family and neighbours and not to leave children or pets in hot cars.

In South Australia, the government has declared a Code Red heat emergency for Wednesday.

This triggers extra funding so services for the homeless can be extended while a special phone line will also operate providing regular checks on the elderly and others at risk from the extreme conditions.